I did that ferry trip as a kid - it was a lot of fun!
The northern part of the Southern Peninsula of Michigan is gorgeous. Be sure to visit Traverse City, Harbor Springs, and Charlevoix on your way (or on your way back). They're seriously nice towns, and very much part of that whole experience. You probably already know that Mackinac doesn't allow motor vehicles, including motorcycles, so you might have to make arrangements to park them.
Back in the day, I knew a number of iron workers who built the Mackinac Bridge.
The Grand Hotel is a nice old place, and there's the touristy Main Street on the island where you used to be able to buy - not kidding - shellac'd horse poop on a wooden stand. I haven't been there since the '90s.
The trip down the western side of Michigan is more engaging and pretty than down the eastern side of the state (just my two cents, I've lived in Michigan all my life).
PS - I never bought the horse poop.